This application is based on and claims priority from Japanese Patent Application No. Hei. 3-106953 filed Feb. 15, 1991, the disclosure of which is incorporated by reference herein.
The present invention relates to a retro-focus type wide-angle lens system suitable for use with cameras, such as single-lens reflex cameras. More particularly, the present invention relates to a wide-angle lens system having an aspheric surface.
In order to ensure adequate back focus, single lens reflex cameras use retro-focus type wide-angle lens systems having a negative power front lens group and a positive power rear lens group. (The terms "group" and "component" are used interchangeably herein to refer to at least one lens element.) For example, Unexamined Published Japanese Patent Application No. 12728/1979 discloses a compact Wide-angle lens system with a simple five-element composition having an overall focal length F=2.8 and a half-view angle of 37.degree.. In another example, Unexamined Published Japanese Patent Application No. 78520/1987 discloses a retro-focus-type wide-angle lens system having a five-element composition, the front lens group of which includes an aspheric surface.
However, the prior art wide-angle lens systems have several problems. The system described in Unexamined Published Japanese Patent Application No. 12728/1979 experiences large off-axis coma and lateral chromatic aberrations, as a result of its simple five-element composition. The system described in Unexamined Published Japanese Patent Application No. 78520/1987 positions an aspheric surface in the front lens group and away from the diaphragm stop to correct for off-axis aberrations, field curvature and distortion. However, this system cannot effectively correct for astigmatism and lateral chromatic aberrations.